Yeah, users can remove themselves from the groups they're in.
I can even vouch for this as someone accidentally gave me Bureaucrat rights and I removed them. When you do it you'll get a little notice saying that you can't remove them since you don't have the rights to, yet the rights will be removed.--H*bad(talk) 19:51, December 21, 2011 (UTC)
Too lazy to login
Sonasaurus — Within its barren layers, something has been held for aeons